The proposed research will investigate "complex" bacterial chemoreceptors. For the enteric bacteria these are defined as receptors having two protein components: a periplasmic substrate-binding protein (BP) and a signal transducer, which spans the cytoplasmic membrane and recognizes substrate-bound BP. The transducer in turn generates a signal that is transmitted through the cytoplasm to the flagella. Four BP-transducer systems are known in E. coli: maltose-BP (MBP) and Tar, galactose-BP (GBP) and Trg, ribose-BP (RBP) and Trg, and dipeptide-BP (DBP) and Tap. Mutational analysis of MBP and Tar has identified within each protein possible sites of interaction. This information has led to the formulation of specific hypotheses about the MBP-Tar pair, which will be examined by site-directed mutagenesis and by measurements of binding affinities between mutant MBP and Tar proteins. The data now available suggest that the MBP-Tar and GBP-Trg interactions are fundamentally different and evolved independently. This prediction will be tested by producing a series of chemotactically defective GBP's that can be compared with mutant MBP's. Peptide chemotaxis involves a receptor system that may prove to be novel in many respects. It is the only known BP dependent chemoreceptor that does not have a sugar as its substrate. Also DBP (55kd) is much larger than the sugar-BP's (30-40kd). The genes coding for the dipeptide transport system will be cloned and their products characterized in an effort to clarify the relationship between dipeptide transport and chemotaxis. The relative effectiveness of individual peptides as chemo-attractants and as ligands for DBP will be compared, and the ability of toxic peptides to serve as attractants will be studied. Finally, a mutational analysis will be initiated to study the DBP-Tar interaction. A pilot project on another tack will explore the utility of GBP-hybrid proteins as vehicles for the export of eukaryotic polypeptides from bacterial cells.